Photoelectric tube and process of making it



Jan. 5, 1937. J. H. DE BOER ET AL 2,066,878

PHOTOELECTRIQ TUBE AND PROCESS OF MAKING IT Filed May 22, 1931 INVENTORS 1m nsmmm de BOER ATTORNEY ?atented Jan. 5, M3?

means PHOTOELECTRIO TUBE AND PROCESS OF MAKING IT Jan Hendrik 'de Boer and Marten Cornelia 'l'evcs. Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application May 22, 1931, Serial No. 539,174 In the Netherlands February 26, 1930 6 Claims. (Ol. 25027.5)

This invention relates to photo-electric tubes of the type which have an electrode containing a photo-electric substance, preferably an alkali metal, which emits electrons when exposed to radiation, the number of electrons emitted depending upon the intensity of the exposure, and

to a process of making such tubes.

It has been proposed to provide between the photo-electric substance and the substratum carrying it, an intermediate. layer containing a chemical compound which is preferably so chosen that no constituent thereof is contained in the substratum and which absorbs the photo-electric substance better than does the substratum. Ad-

vantageously this chemical compound consists of an oxide, or of a halide such as calcium fluoride. With these tubes very favourable results may be obtained.

The principal object of the invention is to improve the properties of these tubes still further.

According to the i nyention the layer containing a chemical comiibuiidiifid of which the photoelectric substance is applied, is substantially mono-molecular. The molecules of this layer seem to have the same orientation, that is to say, to be positioned in substantially the same way, so that the structure of; the layer is extremely uniform, and there is no clotting of the substance constituting the layer which might cause the substratum to be exposed at several points. Probably because of this uniform structure of the intermediate layer, the molecules of the photo-electric substance applied thereto also seem to have the same orientation, so that a very uniformphoto-electric electrode is obtained, by which the electron emission and the constancy of the properties of the tube are very favourably afiected. In addition the electric resistance of the mono-molecular layer is less than that of a thick layer.

Preferably the mono-molecular layer consists of an alkali metal oxide, which appears to givej'f a very high electron emission.

The invention will be more clearly understood 5 by reference to the accompanying drawing, rep- .5 longitudinal section of a photo-electric tube and.

Fi 2 is a detail view of the stem or mount constructed in accordance with the invention, the thickness of the layers being much exaggerated.

The photo-electric tube illustrated has a sealed bulb with a wall lconsisting or glass or 01' a simi- 5 lei-material, to which a stem 2 is sealed on which an electrode 3 is mounted. As seen best in Fig. 2 this electrode 3 comprises a nearlyelosed ring ending in two inner lead wires, which are sealed into the press 4 and are connected to the outer 10 leads 5 and 8 respectively.

The inner wall of the photo-electric tube is coated for the greater part with an extended metallic base or substratum, preferably a thin metal layer 1 consisting, for example, of silver or tungsten which is electrically connected to a leading in wire 8 extending through the glass wall of the tube. The portion of the bulb wall adjacent the wire 8 may be coated with a thin layer or mirror of metal which is applied in a 20 known manner before the stem 2 is sealed in. It is also possible to produce this metal layer by .volatilization after the stem with the electrode 3 is sealed in, by coating this electrode 3 with a quantity of the metal to be applied to the wall 25 of the tube and after exhaustion of the tube. evaporating this metal by means of an electric current supplied by the leads 5 and 5. The metal to be evaporated might also be stretched in form of a wire or hand between the ends of the lead 30 wires of the electrode 3 and then be caused to evaporate by means of a current which is sent through it. When applying the metal layer an open window It! is left by using a shield for a portion of the bulb wail. 35 After the metal layer 1 has been formed, a quantity of halogen or hydrogen halide gas, for example, chlorine or hydrogen chloride, is introduced into the tube, for example, through the tube 9 which may communicate with a vessel 40 containing the gas to be introduced. 01? the gas introduced a mono-molecular layer of halogen or hydrogen halide is adsorbed to the metal layer 1, whereupon the superfluous gas is removed from the tube, for example. by pumping out the excess.

Thereupon the active or photo-electric substance for example, an alkali metal, such as caesium, is introduced into the tube. This may be eflected by evaporating the photo-electric substance from a vessel con "unicating with the so tube 8. Suitable methods for introducing alkali metals into the tube are known in the art. The photo-electric metal contacting with the adsorbed halogen or hydrogen halide acts on this substance thus forming a mono-molecular layer of a halide of the photo-electric substance. For completing this reaction it may be desirable to heat the tube gently. If, for example the caesium contacts with chlorine adsorbed to silver, then it is advisable to heat to somewhat above 100 C., while a tungsten substratum requires a somewhat higher heating. If the adsorbed layer consists of chlorine and the active metal is caesium, then a mono-molecular layer I l of caesium chloride will be formed due to the action of these substances in which'layer the caesium chloride-molecules are present in a definite and substantially uniform orientation.

If the active photo-electric metal is introduced in excess into the tube, the halide formed will be coated with a layer I! of this photo-electric metal. If this metal is not present in excess, a fresh quantity thereof may be introduced into the tube after.

the halide layer has been formed. The photoelectric metal will be partly adsorbed by the mono-molecular halide layer. Preferably the layer of the photo-electric metal is also made very thin, sometimes even mono-molecular, which may be done either by heating the tube in the case 'ofcaesium on a caesium halide layer adsorbed on a silver substratum to about 200 C. for example and simultaneously pumping away the superfluous photo-electric metal or by introducing into the tube a getter to bind the photo-electric metal which is in excess or is not adsorbed by the photo-electric electrode.

If instead of an alkali metal an alkaline earth metal, for example, calcium is introduced into the tube to react with the adsorbed halogen or hydrogen halide layer, a mono-molecular calciumhalide layer is produced, whereupon the superfiuous calcium and the liberated hydrogen, if any, may be removed from the tube. The calcium halide layer thus formed may be coated with an alkali metal by introducing some alkali metal into the tube.

After the production of the metal layer or mirror I in the tube, substances such as, sulfur, hydrogen sulfide, or a volatile sulfur halogen compound, for example, sulfur chloride (SO12) or thionyl chloride (SOClz) may be introduced into the tube, whereupon the substance introduced will be adsorbed in a mono-molecular layer on the metal I. If, for example, an adsorbed hydrogen sulfide layer is formed, and if this layer is subsequently made to contact with caesium or calcium, a mono-molecular caesium or calcium sulfide layer will be formed, whereby hydrogen is liberated. If desired this hydrogen may be removed irom the tube simultaneously with the superfluous caesium or calcium, whereupon the sulfide layer is coated with a layer of photo-electric metal.

Particularly favourable results as regards the electron emission have been obtained with photoelectric tubes in which the mono-molecular layer carrying the photo-electric metal consists of an alkali oxide. When manufacturing tubes of this type one may proceed as follows. To the metal layer 1 consisting, for example, of silver, a monomolecular layer of water is caused to adsorb, which may be effected by introducing some water vapour into the tube, and by removing the superfluous water vapour therefrom. By subsequently introducing an excess of alkali metal, for example, caesium, into the tube, this metal will react with the water adsorbed, which reaction might be represented by the equation:

The last mentioned reaction is facilitated by heating gently. The hydrogen is pumped away from the tube, whereas part of the caesium supplied in excess settles on the caesium oxide layer and is partly adsorbed to the latter. The non-adsorbed caesium may be removed again from the tube in the aforesaid manner.

It will be obvious that this application is not limited to photo-electric tubes in which the monomolecular intermediate layer is produced according to one of the methods described hereinbefore, since this layer may also be formed in other ways. The tubes may be highly exhausted or may be provided with a gaseous filling. Advantageously this gas filling may may consist of a rare gas or of a mixture of rare gases, such as a mixture of neon and argon at a pressure of some millimetres.

Having now described our invention, what we claim and desire to secure by Letters Patent is the following:

1. A photo-electric tube comprising a sealed glass bulb enclosing apair of electrodes, one of said electrodes comprising a silver foundation layer, a sub-stratum consisting of a mono-molecular film of an alkali metal salt adsorbed on the surface of said foundation layer, and a thin layer of said alkali metal adsorbed on said film.

2. A photo-electric tube comprising a sealed glass bulb enclosing a pair of electrodes, one of said electrodescomprising a metallic foundation layer having a silver surface, a mono-molecular film of a salt of a photo-electric metal adsorbed on the surface of said foundation layer, and a thin layer of said photo-electric metal adsorbed on said film.

3. A photo-electric tube comprising a sealed glass bulb enclosing a pair of electrodes, one of said electrodes comprising a silver foundation layer, a mono-molecular film of an oxide of a photo-electric metal adsorbed on the surface of said foundation layer, and a thin layer of said photo electric metal adsorbed on said film.

4. A photo-electricitube comprising a sealed glass bulb enclosing/a pair of electrodes, one of said electrodes comprising a silver foundation layer, a mono-molecular film of an oxygen salt of caesium adsorbed on the surface of said foundation layer, and a thin layer of caesium adsorbed on said film.

5. The process of forming an electrode for photo tubes having an extended silver foundation layer inside the tube which comprises introducing water vapor into the tube, removing the excess water vapor, introducing caesium vapor into the tube, heating the tube gently while it contains caesium vapor, and removing the excess caesium vapor by evacuating the tube.

6. A photoelectric tube comprising a sealed glass bulb enclosing a pair of electrodes, one of said electrodes comprising a metallic foundation layer, a mono-molecular layer of an alkali metal salt on the surface of said foundation layer, and a mono-molecular layer of the said alkali metal on the surface of said layer of salt.

JAN HENDRIK DI. BOER. MAR'I'EN CORNELIS 'I'EVES. 

